This invention relates to a process of forming a metal or metal compound coating on a face of a heated glass substrate during its displacement in a given direction along a tunnel by contacting the substrate at a coating station in said tunnel with at least one stream of droplets comprising a substance or substances from which said coating metal or metal compound is formed on said face, and simultaneously exerting suction forces in exhaust ducting to draw environmental gases away from that station.
Such a process can be employed for coating glass sheets and for coating a continuous freshly formed ribbon of flat glass. In the latter case the coating of the glass can take place in the usual annealing gallery or lehr. The process can be used for forming coatings, e.g. metal oxide coatings, which modify the apparent colour of the glass and/or which have some other required properties in respect of incident radiation, e.g. an infra-red-reflecting property.
The suction forces which in known processes are exerted downstream from, i.e. forwardly of, the coating station promote the formation of coatings of homogenous structure with uniform coverage of the substrate.
When applying such a process the best results have been obtained when using a stationary exhaust ducting whose inlet(s) extend(s) transversely across the path of the glass substrate. Suction forces can then be maintained over the full transverse extent of the coating as it is formed.
Even in those circumstances however, defects sometimes occurred internally of the coating and/or at the glass or coating surface and of a nature which suggests they are attributable to products which have been formed in the environment or to impairment of the coating during its formation by contact with a coating precursor compound which has not reacted or has not completely reacted at the coating station.
The foregoing defects may not make the product valueless but may disqualify the product for top quality rating and this becomes more likely with increase in the quality standards demanded by customers. If the defects are at the surface of the coating, the quality of the product can be in some but not all cases be improved by an after-coating surfacing treatment, but of course such addition treatments add to the product costs.
Attempts have been made to promote coating quality by careful control of the suction forces to ensure that they are effective for their intended purpose but do not disturb the steadiness of the droplet stream(s). Improvements have also been sought by modifying the number and the arrangement of the exhaust ducts. A further measure which has been proposed is the employment of two or more exhaust ducts disposed at successive spaced positions downstream from the coating station so that gas which flows downstream past one such duct can enter the next one. Another proposal is to provide successive exhaust ducts at different levels above the path of the glass substrate. These measures, while useful, have not been found to be a sufficient solution to the problem of coating defects above referred to.